1. Field of the Invention
The present invention relates to integrated circuits (ICs), temperature sensors, and in particular, to a temperature sensor for use with low voltage power supply circuits.
2. Description of the Related Art
The base-emitter voltage VBE of a forward-biased transistor is a fairly linear function of absolute temperature T in degrees Kelvin (.degree. K.), and is known to provide a stable and relatively linear temperature sensor. Proportional To Absolute Temperature (PTAT) sensors eliminate the dependence on collector current by using the difference .DELTA.VBE between the base-emitter voltages VBE1 and VBE2 of two transistors that are operated at a constant ratio between their emitter-current densities to form the PTAT voltage. The emitter-current density is conventionally defined as the ratio of the collector current to the emitter size. Thus, the basic PTAT voltage .DELTA.VBE is given by: EQU .DELTA.VBE=VBE1-VBE2 (1) EQU .DELTA.VBE=(kT/q)* ln(J1/J2) (2)
where k is Boltzmann's constant, T is the absolute temperature in degreed (Kelvin), q is the electron charge and J1 is the current density of a transistor T1 and J2 is the current density of a transistor T2. As a result, when two silicon junctions are operated at different current densities (J1, J2), the differential voltage .DELTA.VBE is a predictable, accurate and linear function of temperature.
The basic PTAT voltage is amplified so that its gain, i.e., its sensitivity to changes in absolute temperature, can be calibrated to a desired value, suitably 10 mV/.degree. K., and buffered so that a PTAT voltage can be read out without corrupting the basic PTAT voltage. A temperature sensor embodying such technology is the LM135 Precision Temperature Sensor, available from National Semiconductor Corporation. Such temperature sensors when biased from a nominal source of current develop a 10 mV/.degree. K. voltage response, operate over the range of -55.degree. C. to 155.degree. C., and when calibrated at 25.degree. C. have less than a 1.degree. C. error over a 100.degree. C. range. To obtain a Fahrenheit or Celsius scale reading the output of a sensor is combined with the output of a precision temperature-stable voltage that is designed to be equal to the temperature sensor at the temperature scale's zero point. This is an undesirable approach because it requires a sensor along with a number of other stable, low-drift external components.
It is well-recognized that a single IC chip could be provided with the circuits necessary to develop both a temperature-related voltage and a temperature-stable precision reference voltage. However, this would require a very complex IC design.
FIG. 1 illustrates a conventional temperature sensor 100 that provides an output voltage Vout scaled Proportional To Fahrenheit Temperature (PTFT). Thus, output voltage Vout of PTFT sensor 100 rises in proportion to changes in Fahrenheit temperature. As shown in FIG. 1, conventional n-p-n transistors QA, QB have a 10:1 emitter area ratio and generate a large PTAT voltage VPTAT of about 1.59 V at room temperature. This characteristic is shown as curve 41 of the graph in FIG. 4. The base-emitter voltages of conducting transistors have a negative temperature coefficient, shown as curve 42 of FIG. 4. Therefore, the two base-emitter voltages VBEs of transistors QB, QC are subtracted from the large PTAT voltage VPTAT to shift the voltage VPTAT by an offset voltage. The resulting voltage is amplified by non-inverting amplifier A2 to provide an output voltage Vout that is linearly proportional to Fahrenheit temperature. This characteristic curve is shown as curve 43 of FIG. 4.
With the 10:1 emitter ratio shown, at 77.degree. F. the two transistors QA, QB require a 60 mV (VPTAT) offset to be imposed across R1. To enforce this condition, amplifier A1 will servo the base of transistor QA to a level of n * 60 mV, also voltage VPTAT. A value of 26.5 is chosen for n so that at 77.degree. F. the voltage across resistor R1 is 1590 mV with a slope of 2.963 mV/.degree. F. Then from this voltage the two base-emitter voltages VBEs of transistors QA, QB are subtracted. Their 77.degree. F. value of (588.2 mV-1.2032 mV/.degree. F.) each provides a 77.degree. F. result of 413.5 mV plus 5.37 mV/.degree. F. at the positive input of non-inverting amplifier A2. When this voltage is amplified by a gain of 1.862, the output voltage Vout at 77.degree. F. will be 770 mV with a gain of e.g., 10 mV/.degree. F. If there is an error in the output voltage Vout at any particular temperature, this error can be fixed by adjusting the ratio n. In this manner, the offset voltage is effectively subtracted so that the output voltage Vout of PTFT sensor 100 is 0 V at 0.degree. F. and is linearly proportional to Fahrenheit temperature, having a slope of 10 mV/.degree. F.
The conventional PTFT sensor 100 of FIG. 1 has several drawbacks. Such sensor 100 requires relatively large supply voltages to respond over the desired operating range and to supply any overhead voltage needed to operate the sensor. Over the past decade, there has been a trend toward reducing the supply voltage which has gradually decreased from 5 Volts to 2.5 and is now even as low as approximately 1 Volt. Thus, products which run off lower voltage supplies cannot use PTFT sensors of the type shown in FIG. 1. Even low-voltage sensors having a regulator configuration are unacceptable due to the Early Effect, a change in collector current with a change in collector voltage.
Another drawback is that nonlinearities occur on the order of 1 to 3 percent over a 360 degree Fahrenheit range. Although additional circuits can be added to temperature sensor 100 to cancel these nonlinearities, such circuits require additional voltage.
Thus, a need exists for a temperature sensor that operates on a wide range of supply voltage, from approximately one to twelve volts. In addition, such temperature sensor should operate without the occurrence of uncorrected nonlinearities.